1. Field of the Invention
The present invention relates to a connector device used to couple a turbomolecular pump (TMP) to a vacuum chamber to be pumped and, more particularly, to a TMP connector device capable of sufficiently suppressing transmission of vibrations of relatively low frequencies produced by a TMP to a vacuum chamber.
2. Description of Related Art
Apparatus that require high vacuum such as electron microscope and charged particle beam lithography systems employ vacuum pumps such as oil diffusion pump (DP) and turbomolecular pump (TMP). In recent years, oil used in oil diffusion pumps has been seen as a problem because the oil is released as a vapor into a vacuum to thereby contaminate objects to be inspected or machined.
Therefore, in order to obtain a clean vacuum, TMPs have been increasingly used. As is well known in the art, a TMP performs vacuum pumping by expelling gaseous molecules by turbine blades mounted to a rotor that rotates at high speed. Consequently, vibrations of the rotational frequency of the rotor are produced. However, vibrations generated from the TMP are not limited to this type of vibrations. A resonance having the natural frequency of the turbine blades brings about vibrations. In addition, in a case where a rotor is supported by a magnetic bearing to provide low vibrations, positional control of the rotor results in vibrations.
FIG. 5 shows a conventional structure of a TMP. This pump 1 has a casing 30 that is a closed container. A rotor 31 to which a multiplicity of turbine blades are mounted is accommodated within the casing 30. The casing 30 has an intake port 32 that opens into its end surface lying in the direction of extension of the axis of rotation 0 of the rotor 31. Furthermore, the casing 30 has an intake flange 2 for connecting the intake port 32 to an apparatus (such as an electron microscope) to be pumped. An outlet port 33 is formed in the side surface of the casing 30.
The intake flange 2 is coupled to a flange 3 of the body of the apparatus into which the outlet port of the apparatus to be pumped opens, via a bellows 4 that is a connective exhaust tube. The bellows 4 has flanges on both ends for connection with the flanges 2 and 3. A vibration-absorbing member 5, as made of rubber, is disposed around the bellows 4. The vibration-absorbing member 5 and the bellows 4 cooperate to constitute a vibration isolator 6. When the bellows 4 is mounted to the intake flange 2 and to the flange 3 of the body, O-ring seals 9a and 9b are sandwiched and clamped between the flange 3 of the body and the flange of the bellows using clamps 7 and 8 (clamps a and b).
FIG. 6 shows a spectrum of vibrations produced by a TMP. Frequency (in Hz) is plotted on the horizontal axis on a logarithmic scale. Acceleration is plotted on the vertical axis on a logarithmic scale. Vibrations are produced in both horizontal and vertical directions as viewed in FIG. 5. In the following description, only vibrational components in the vertical direction are treated. The same principle applies to vibrational components in the horizontal direction. A vibrational peak 11 of a TMP rotational frequency component is observed at a rotational frequency fR (from about 600 Hz to 1 kHz or higher) which differs according to the type or manufacturer of the TMP.
The component due to the resonance at the natural frequency 12 of the turbine blades is observed at the natural frequency fB of the turbine blades. Generally, this is lower than the rotational frequency fR of the TMP and on the order of 200 Hz. Furthermore, consideration is given to prevent the frequencies fR and fB agree; otherwise, great vibrations would take place. Where the rotor is supported by a magnetic bearing, control of the position of the rotor induces vibrations. In the example of FIG. 6, for the sake of simplicity of illustration, this is shown as a wide-band component 13 having a constant amplitude of A in a region of tens of Hz or higher. In practice, some TMPs show sharp peaks rather than components covering wide-frequency ranges.
FIG. 7 shows an example of vibration transfer function in the vertical direction of a configuration having the vibration isolator 6 shown in FIG. 5. Frequency (in Hz) is plotted on the horizontal axis on a logarithmic scale. Vibration transmissibility (in dB) is plotted on the vertical axis on a logarithmic scale. The mass of the TMP 1 and the spring constant in the direction of elongation and contraction of the vibration isolator 6 together form a vibration system. The transmissibility increases due to the resonance amplification at the resonant frequency fC of the vibration system. As the frequency is increased further, the transmissibility drops.
FIG. 8 shows a spectrum of vibrations which are produced by a TMP and shown in FIG. 6 when the vibrations are transmitted to an apparatus to be pumped via the vibration isolator 6 having the vibration transfer function shown in FIG. 7. Frequency (in Hz) is plotted on the horizontal axis on a logarithmic scale. Acceleration is plotted on the vertical axis on a logarithmic scale. Because of the aforementioned characteristics, i.e., the transmissibility decreases with increasing frequency, a large proportion of the TMP rotational frequency component 11 is removed at the TMP rotational frequency fR that is a relatively high frequency. The amount of vibrations transmitted to the flange of the body is suppressed sufficiently. However, vibrations of the natural frequency fB of the turbine blades that is a relatively low frequency are relatively large. Furthermore, the vibration transmissibility of the low-frequency, wide-band component 13 induced by the control of the magnetic bearing is relatively large.
A small vibration isolator of this type is disclosed (see, for example, in JP-A-2004-360784 (paragraphs 0014-0018; FIGS. 1 and 2)), and includes a first covering having a bottomed peripheral wall and a second covering having a bottomed peripheral wall that is smaller in diameter than that of the first covering. The first and second coverings are disposed opposite to each other such that their peripheral walls are made to overlap each other to form an interior covering space. A coil spring that biases the first and second coverings away from each other to support the static load of an object which should be made vibration-free is mounted in the covering space. Also, a pillar-like viscoelastic member is mounted in the coil spring (hence within the covering space) coaxially with the coil spring to attenuate vibrations by compressive deformations and tensile deformations in the direction of the axis.
Additionally, a pumping device is known which has a pump flange coupled to the pump, an apparatus to be pumped, an apparatus flange coupled to the apparatus, a bellows mounted between the apparatus flange and the pump flange, and a rubber member mounted between the apparatus flange and the pump flange (see, for example, in JP-A-2008-232029 (paragraphs 0045-0048; FIGS. 1 and 2)). There are n grooves formed in the outer periphery of the bellows. Parts of a resilient material are disposed in m of the n grooves.
Further, a charged particle beam system having a charged particle beam instrument including an electron optical system for directing an electron beam or ion beam at a target, a vacuum pumping system having a suction pump for evacuating the inside of the instrument, a suction path for placing the instrument in communication with the vacuum pumping system, a vibration-isolating portion placed in the suction path, and a flexible path member mounted in the vibration-isolating portion (see, for example, in JP-A-2007-165232 (paragraphs 0012-0034; FIG. 2)). This system has a contraction-hindering means which suppresses the flexible path member from contracting in such a direction that the instrument and the vacuum pumping system are drawn toward each other by suction of the vacuum pumping system.
As described previously, in some cases, low-frequency vibrations are not sufficiently removed but transmitted to the body flange, thus adversely affecting the apparatus to be pumped. Furthermore, it is conceivable to arrange plural vibration isolators in series to enhance the vibration-removing rate. In this case, a long vibration-removing assembly is built, so that this structure cannot be applied to the case where a sufficient space cannot be secured in the height direction.